Posttreatment of phosphatized metal surfaces with silicates

ABSTRACT

A method of posttreatment of phosphatized metal surfaces which comprises applying to phosphate layers applied to metal surfaces, an aqueous alkali metal silicate solution having a mol proportion of SiO2 to alkali metal oxide selected from the group consisting of Na20 and K2O of from 5 to 1 to 10 to 1 and a SiO2 concentration of from 0.01 to 1 percent by weight, said alkali metal silicate solution containing a water-soluble quaternary nitrogen compound having at least one nonhydroxylated alkyl group on the quaternary nitrogen atom selected from the group consisting of mono- and polyquaternary nitrogen compounds, said quaternary nitrogen compound being present in a ratio of SiO2 to quaternary nitrogen compound calculated as quaternary ammonium oxide of from about 35 to 1 to about 1000 to 1, and drying said posttreated phosphatized metal surfaces.

106-74. AU 112 E v-wu ounce talent 3,615,895

[72] lnventon Hehlut Vol Freybold FOREIGN PATENTS Mort-W; 566306 121944 0 1B tai I48 6.15R Velker Welle, Hilda, m Ger-any n n I [2]] App]. No. 5 190 Pnmary Examiner-Ralptt S. Kendall [22] Filed Sept 8' 1969 Attorney-Hammond & Lxttell [45] Patented Oct. 26, 1971 [73] Auignee Huh! I: Ch GIbH W, Ger-any [32] Priority Sept. 16, 1968 [33] Alatria [3 1] 9013/ ABSTRACT: A method of posttreatment of phosphatized metal surfaces which comprises applying to phosphate layers [54] or "s"; Am! METAL applied to metal surfaces, an aqueous alkali metal silicate SURFACES m SILICA-[Es solution having a mol proportion of S1 to a a 1 metal oxtde 8 cm Dn'hp aelected from the group consisting of Na,0 and K,O of from 5 to I to 10 to l and a $10, concentration of from 0.01 to l pers cent by weight, said alkali metal silicate solution containing a "7/1351, 148/6-l5 Z, 106/74 water-soluble quaternary nitrogen compound having at least 7/10 one nonhydroxylated alkyl group on the quaternary nitrogen on R, tom elected from the group consisting of mono. and 3 I06/"4 polyquatemary nitrogen compounds, said quaternary nitrogen a cm compound being present in a ratio of SiO, to quaternary nitrogen compound calculated as quaternary ammonium UNrrED STATES PATENTS oxide of from about to l to about 1000 to l, and drying said 3,267,056 8/1966 lhde et al. l 17/ l 35.] X posttreated phosphatized metal surfaces.

POS'I'I'IIATMINT OI HIMA'I'IZID METAL SURFACES WITH SILICA'I'ES THE PRIOR ART Forsometime,aprocesshasbeenknownbywhichoneapplies phosphate layers to metal surfaces in order to supply metal surfaces with a higher resistance against environmental influences, such as corrosion. This process is commonly called a "phosphatizing" process. Moreover, the phosphate layers serve to improve lacquer adherence and, optionally, to facilitate cold forming of the metal. In addition, they are useful for electrical insulation purposes. In many cases, these phosphate layers on metal surfaces are submitted to an aftertreatment for the improvement of their properties. Frequently, the after-treatment is carried out by means of agents which contain acid chromium Vl compounds. The disadvantage of such a chromic acid treatment lies in the toxicity of the treated surface, for example, for use as containers of foodstufl's. Also, waste water problems arise which require additional precautionary measures.

Thus,ithasalready been suggestedsometimeagotoundertake in certain cases an after-treatment of the phosphate layers with alkali solutions, particularly sodium silicate solutions. An essential disadvantage, however, of this known method. is the strong alkalinity of the treated surfaces which, among other disadvantages, requires an intensive after-rinsing of the metal surfaces with water.

OBJECTS OF THE INVENTION An object of the present invention is the development of a simple and economical method of improving the properties of phosphatized metal surfaces by a posttreatment thereof.

Another object of the invention is the development of a method of posttreatment of phosphstmed metal surfaces which comprises applying to phosphate layers applied to metal surfaces, an aqueous alkali metal silicate solution having a mol proportion of SiO, to alkali metal oxide selected from the groupconsistingofNqOandLOoffromStol to lOtol and a SiO, concentration of from 0.0l to 1 percent by weight, said alkali metal silicate solution containing a water-soluble quaternary nitrogen compound having at least one nonlrydroxylat'ed alkyl group on the quaternary nitrogen atom selected from the group consisting of monoand polyquaternary nitrogen compounds, said quaternary nitrogen compound being present in a ratio of 8K), to quaternary nitrogen compound calculated as quaternary ammonium oxide of from about3$to l toabout l000to l,anddryingsaidposttreated phosphatized metal surfaces.

These and other objects of the invention will become more apparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION It has now been found that the above objects can be achieved and the working method used thus far can be improved while avoiding the indicated disadvantages to a large extent if the method of after-treatment of the phosphate layers on metal surfaces with alkali solutions according to the invention is employed.

The new process is characterized in that the phosphate layers as applied to metal surfaces are post treated with an aqueous alkali metal silicate solution, whose mol proportion ofSi0,toalkalimetaloxideisfrom Sto l to lto l andits SiO, concentration in the solution is from 0.01 to I percent by weight. The alkali metal silicate solutions of the invention contain a water-soluble, monoor polyquaternary nitrogen compound having at least one nonhydroxylated alkyl on the quaternary nitrogen atom in a ratio of SiO; to said quaternary nitrogen compound calculated as quaternary ammonium oxide of from about 35 to l to about 1000 to l. Optionally, they may also contain from about 0.5 to 25 percent by weight, based on the SiO, content of the alakli metal silicate solution,

and/orfromabout0.05to2 percentbyweight, basedoa the SiO, content of the alkali metal silicate solution of known water-soluble corrosion inhibitors. After treatment with the alkali metal silicate solution for a period of from about 30 seconds to about 5 minutes at temperatures of from about 20' C. to I00 C., the treated metal surface is dried. No rinsing of the treated metal surface is required before drying. Obviously, the phosphatized metal surfaces can remain in contact with the alkali metal silicate solutions for longer periods than those indicated above, but no improvement in results occurs after a contact time of about 2 minutes.

The production of the phosphate layers on the metal surfaces is elected according to customary methods. It is known, for example, to produce iron phosphate layers on metal surfaces by treating the iron surfaces with solutions which contain phosphoric acid or acid ammonium and/or acid sodium phosphates. Generally, the pli value of these solutions is "between 3 and 6. These phosphatizing solutions, furthermore,

can contain known oxidation agents, such as chlorates, chromates, nitrites, nitrates, hydrogen peroxide, or organic nitro compounds such as, in particular, nitroguanidine as accelerators. Also heavy metal compounds of cobalt and nickel can be used as further additives. Moreover, phosphatizing baths can also contain complex formers, such as aminopolycarboxylic acids, and, in particular, anhydrous phosphates, such as pyrophosphstes, hexametaphosphates or tripolyphosphates.

In addition, such phosphate layers on metal surfaces can be treated by the process of the invention, which have been prepared with phosphatizing baths, which contain layer forming cations, such as zinc, manganese or calcium, individually or in mixtures. The phosphate layers can be produced on iron sheets,sincorgalvanisedironshsetsand in somecasesalsoon aluminum. A

A molproportion ofSiO,toalkali metal oxide ofS to "M is usually not achieved with commercial alkali metal silicate solutions. However, such alkali metal silicate solutions are obtained, ifone adds to an alkalimetal silicate solution ofa low molar ratio of Slo to alkali metal oxide, at least one watersoluble monoandlor polyquaternary nitrogen compound which has attached to the quaternary nitrogen atoms at least one nonhydroxylated alkyl group. The quaternary nitrogen compound is added in such an amount that the molproportion of SiO, quaternary nitrogen compound (calculated as ammonium oxide) amounts to from l,000:l to 35:]. The amount of quaternary nitrogen compound may be in excess of the 35:1 ratio without effecting the stability of the solutions thus obtained. Subsequently, in a known manner, the mol proportion of SiO, to alkali metal oxide is adjusted to a value between $:l and 10:1.

Generally, commercial alkali metal silicate solutions, based on sodium or potassium are preferred, which have a mol proportion ofSiO,:Na, O or K, O ofbetween 2:! and 4:l as staning substances. Also mixtures of sodium and potassium silicate solutions can be used.

The mono and/or polyquaternary nitrogen compounds in solid or liquid form or as aqueous solutions are added to the alkali metal silicate solutions. Also salts of the quaternary nitrogen compounds, such as the chlorides, sulfates or nitrates can be used.

A preferred method consists in that quaternary nitrogen compounds are added in the form of their hydroxides since thus solutions of the lowest are obtained.

The water-soluble quaternary nitrogen compounds must have attached to the nitrogen atom at least one alkyl radical which is not substituted by hydroxyl groups. The remaining three groups which are bonded to a nitrogen atom can consist of alkyl or hydroxyalkyl radicals which are the same or different These alkyl or hydroxyalkyl radicals can be straight or branchedchaimandthecarbonchalnseanbeinterrupted by hetero atoms, such 8 oxygen or nitrogen. Moreover, two groups can be bonded to the nitrogen ring in a cyclic manner. The total number of the carbon atoms of the quaternary of a water-soluble salt of an organic polycarboxylic acid nitrogen compounds can vary over a wide range and a only limited in that the compounds must be water-soluble. Quaternary nitrogen compounds with a long-chained radical having up to 18 carbon atoms can be utilized. In the case ofmore than one long-chained radical being attached to the quaternary nitrogen atom, the chain length is usually restricted to l2 carbon atoms. The quaternary nitrogen compounds according to the invention can have one or more quaternary nitrogen atoms, and mixtures of various quaternary nitrogen compounds may also be utilized.

The quaternary nitrogen compounds utilized are preferably compounds of the formulas:

[ (BMPA' ummmommn 2A- and wherein R is a member having from one to IE carbon atoms selected from the group consisting of alkyl and alkylol wherein at least one R is alkyl and the total number of carbon atoms in the R's in any compound does not exceed 30; R, is a member selected from the group consisting of alkylenc having from two to l2 carbon atoms and cyclohexylene; R, is a member having from one to six carbon atoms selected from the group consisting of alkyl and alkylol; and A is a member selected from the group consisting of hydroxyl anions, chloride anions, sulfate anions, nitrate anions and anions of organic polycarboxylic acids. Table 1, lists representative quaternary nitrogen compounds which are utilizable in the preparation of the silicate solutions.

TABLE 1 Table l-Continucd Generally, because of the easy accessibility, sodium silicate solutions are preferred. Particularly, such solutions are taken into consideration whose mol proportions of SiO,:Na,O are 9 to 9: l.

Following the addition of the quaternary nitrogen compounds, the mo] proportion of SiOgNuO or K,O is brought up to the desired value ofS to 10:1, preferably 6 to 9:1. The adjustment of the mo] proportion can be eflected by the admixing of SiO, to the solution. The Si(), has to be admixed in a form which is soluble in aqueous alkali metal silicate solutions as, for example, finely divided silicic acid, silicic acid sols or gels.

be adjusted by decreasing or removing the alkali metal ions. This can be accomplished by the reduction of the free alkali metal ions titrable against methyl red, which are available to silicic acid and cause the alkalinity of the solutions.

The molar ratio is understood to mean the ratio of SiO, to free alkali metal oxide. Thus, it is possible to bind the alkali metal ions to the desired degree with acids, such as sulfuric acid, hydrochloric acid or nitric acid, and these reduce the alkalinity. In spite of the added foreign ions, the viscosity of the solutions does not increase substantially thereby.

Moreover, the alkali metal ions can also be removed by ion exchangers from the alkali metal silicate solutions after the addition of the quaternary nitrogen compounds. in the preferred procedure, it is particularly advantageous that the treatment with ion exchangers can be effected in concentrated solutions of more than 10 percent SiO, without inactivating the ion exchanger. Thus, the generally difficult reconcentration of the alkali metal silicate solutions is not required.

The above-described preparation of the alkali metal silicate solution utilized in the process of the invention is further described in French Pat. No. 1,551,442.

Generally, the alkali metal silicate solutions prepared in such a manner, with a high portion of SiO,, are diluted with water to such a degree that the SiO, concentration of the solution used in the posttreatrnent of phosphatized metal surfaces is from 0.01 to l percent by weight. Preferably, these solutions are utilized in SiO, concentrations of from 0.02 to 0.2 percent by weight.

Furthennore, it was found that the subsequent sealing effect of the phosphate layers on the metal surfaces can be still more improved if the alkali metal silicate solutions utilized contain an addition of a water-soluble salt of a polycarboxylic acid. Aliphatic, cycloaliphatic or aromatic carboxylic acids can be utilized in the form of their water-soluble salts with alkali metals, ammonium salts and, in particular, quaternary ammonium salts. The organic radicals attached to the quaternary nitrogen atom can be the same with quaternary salts as with the quaternary nitrogen compounds already described above as additions to the silicate solutions.

The water-soluble salts of these acids can be applied individually or in a mixture. if so desired, water-soluble salts of polycarboxylic acids can also be employed which contain N0 -Nl-l, as well as, in particular, -Ol-l groups. Examples of organic polycarboxylic acids are particularly alkanedioic acids such as oxalic acid, malonic acid, succinic acid, alkenedioic acids such as maleic acid, fumaric acid; hydroxyalkanepolycarboxylic acids such as citric acid, malic acid, gluconic acid; phthalic acid; aminoalkanedioic acids such as aaminoadipic acid; heterocyclic dicarboxylic acids such as 4- amino-pyridine-2,6-dicarboxylic acid; and S-nitroisophthalic acid. It is practically technologically the same to employ in- However, the mo] preparation SiO,:Na,O or K,O can also stead of salts, also the free polycarboxylic acids since they are convened by the alkali metal solutions into salts.

The water-soluble salts of the polycarboxylic acids are utilized in an amount of 0.5 to 25 percent by weight, based on the SiO, content of the alkali metal silicate solution. Preferably, amounts of 2 to percent by weight based on the SiO, content are used.

The sealing effect of the phosphate layers on the metal surfaces finally can still be further improved by the addition of known corrosion inhibitors, such as sodium benzoate, pnitrophenol, hydrazine hydrate, or antimony compounds, preferably in amounts of from 0.05 to 2 percent based on the SiO, content of the solution.

The metal surfaces having a phosphate layer are submersed in the described alkali metal silicate solutions or sprayed with them. The drying can be effected by air or by a slight warming.

The posttreatment of phosphate layers with alkali metal silicate solutions can be carried out at temperatures of to 70 C. Suitably, however, temperatures of 70 to I00 C. are employed. Upon adding the above-mentioned water-soluble salts of polycarboxylic acids, one obtains very good results, however, upon a treatment of l to 2 minutes and at temperatures of 40 to 70' C. Also, in the last-mentioned case, the application concentration of the silicate solution may be lowered. Furthermore, a prevention of the crater formation on subsequent electrophoretic lacquering is achieved.

In general, the advantages of the new process are the following: upon a low concentration on one hand, and upon a high proportion of SiO, alkali metal oxide on the other hand, a favorable subsequent sealing of the phosphate layers takes place, without damage caused by alkali, particularly damage to layer applied lacquers does not occur. Such an efl'ect is not achieved, for example, if the posttreatment is made with a silicic acid sol.

The following examples are illustrative of the practice of the invention without being limitative in any manner. The amounts given by the following examples are in percent by weight unless otherwise noted.

EXAMPLE 1 PREPARATION OF THE ALKALI METAL SILICATE SOLUTIONS The alkali metal silicate solutions having a high SiO, content, as utilized in the posttreatment of phosphatized metal surfaces were prepared as follows:

PREPARATION A A solution of sodium silicate containing 22.1 of SiO, and having a mol ratio of 3.9 Si0,: lNa,O was mixed with hexamethyl-hexamethylene diammonium hydroxide in an amount calculated to give a mol ratio of silica to quaternary nitrogen compound based on the total SiO, content of the final solution of 850 SiO,: 1 quaternary ammonium oxide. In this case, 655 g. of the sodium silicate was mixed with the required amount of quaternary nitrogen compound and the mixture was heated to boiling. In order to raise the SiO, ratio, 63 g. of finely divided precipitated hydrated silica with an SiO, content of 87.5 percent was added. The mixture was heated to the boiling point and continuously stirred until the solution became clear. The reaction mixture was then cooled to 50-60 C. and 200 g. of water was added. A clear, stable sodium silicate solution having a low viscosity of 40-50 cp was obtained. The mol ratio was 5.2 SiO, l Na,0 and the solution contained 2L8 percent of SiO,. For use in the process of the invention, the alkali metal silicate solution was diluted with water to give an SiO, content of 0.2 percent.

PREPARATION B 593 g. of sodium silicate having 22.] percent of silica and a ratio of 3.9 SiO, lNa,O was mixed with 48 g. of an aqueous solution containing 31 percent of tetramethylammonium hydroxide. The mixture was heated to boiling. 96.2 g. of the finely divided silica mentioned in Preparation 0 was added, and the heating was continued until the system clarified. The mixture was then diluted with I62 g. of water and a stable sodium silicate solution having a low v'ucosity and a mol .ratio of 6.4 SiO, l Na,0 and 24 percent SiO, was obtained. The mol ratio of SiO, in the final solution to quaternary nitrogen compound calculated as ammonium oxide was 44 SiO, l quaternary ammonium oxide. For use in the process of the invention, this alkali metal silicate solution was diluted with water to give an SiO, content of 0.5 percent.

PREPARATION C 1000 g. of sodium silicate having 22.l percent of silica and a mol ratio of 3.9 SiO, l Na,0 was mixed with tetraethylammonium hydroxide in an amount such that the mol ratio obtained was SiO, to l quaternary nitrogen compound calculated as quaternary ammonium oxide. The mixture was heated to C. Finally, 25 g. of concentrated sulfuric acid, diluted 1 part of sulfuric acid to 4 of water, was added dropwise under strong agitation. During this addition a precipitate formed but redissolved in a short period of time. A sodium silicate solution containing [9.4 percent of silica and having a mol ratio of 5.4 SiO, to 1 mol of free Na O as determined by titration against methyl red, was obtained. The solution was clear. had a low viscosity, and was stable for a long period of time. For use in the process of the invention, the solution was diluted with water to give an SiO, content of 0. l percent.

PREPARATION D 500 g. of a potassium silicate solution containing 20.2 percent of SiO, and having a ratio of 3.2 SiO, to K,O was mixed with enough tetraethylammonium hydroxide to give a mol ratio of 92 SiO, l quaternary nitrogen compound calculated as quaternary ammonium oxide, and then 250 g. of a strongly acid ion exchange resin in the hydrogen form was added with stirring. This system was filtered after 20 minutes and the potassium silicate solution obtained was stable, had a low viscosity, a silica content of 18.7 percent, and a mol ratio of 4.5 SiO, l K,O. For use in the process of the invention, this solution was diluted with water to give an SiO, content of 0.2 percent.

PREPARATION E 500 g. of sodium silicate solution containing 30.5 percent of SiO, and having a mol ratio of 3." SiO, :Na, 0 was mixed with sulficient hexamethyl dodecamethylene diammonium hydroxide to give a ratio of I36 SiO, l quaternary nitrogen compound calculated as quaternary ammonium oxide, based on the total silica content of the final solution. Then 500g. of a silica sol containing 30 percent of SiO, was added with strong agitation. The sodium silicate solution obtained was clear and stable. It had low viscosity, a mol ratio of 6.28Si0, lNa, O and a silica content of 30.2 percent. For use in the process of the invention, this solution was diluted with water to give an SiO, content of 0.2 percent.

PREPARATION F Practically the same results are obtained if instead of the quaternary ammonium compounds named in the examples A to E to quaternary ammonium compound of another kind, as listed in table I above, is used, always in equimolar quantity. Instead of the hydroxides the corresponding salts, as for example the chlorides, sulfates or nitrates can be used.

PREPARATION G Oxalic Acid Malonic Acid Maleic Acid Fumaric Acid Succinic Acid Citric Acid Malic Acid Gluconic Acid Phthalic Acid a-Aminoadipic Acid 4'-Aminopyridine-2,6-diearboxylic acid S-Nitroisophthalic Acid Technically equivalent to these are the ammonium, potassium, and sodium salts.

It is also technically equivalent if, instead of the quaternary bases on the one hand, and the polybasic carboxylic acid on the other hand, named in the description and the examples, the corresponding salts of the two compounds are used as additives to the alkali silicate solutions prior to adjusting the SiO, ratio.

EXAMPLE 2 Iron sheets were treated for 10 minutes at 95' C. in a phosphatizing solution which contained 9 g. of Mn, 27.3 g. of P, 0,, 2.9 g. of NO, and 0.4 g. of sodium hexametaphosphate per liter. Subsequently, the sheets now covered with a manganesephosphate layer were rinsed with water and then treated for 2 minutes at approximately 65' C. with a sodium silicate solution. This sodium silicate solution contained approximately 0.68 percent by weight Sit), and a mol ratio of SiO, to Na, of 6.4:]. The solution also contained a quaternary ammonium compound according the example l, table 1, in an amount which corresponded to a mol ratio SiO,: quaternary ammonium compound, calculated as ammonium oxide, of 44: 1. In addition, the alkali metal silicate solution contained percent malonic acid as well as 2 percent sodium benzoate, each based on the SiO, content of the solution. After treatment, the iron sheets were rinsed with water and dried. A phosphate coating with good corrosion protection was obtained.

EXAMPLE 3 Zinc sheets were treated for 5 minutes at 96' C. in a phosphatizing solution which contained 2.4 percent monozinc phosphate, 37.6 percent total phosphoric acid, l5.6 percent free phosphoric acid, l2.5 percent zinc oxide and 2.] percent acid manganese phosphate per liter. Subsequently, the sheets covered with a zinc phosphate coating were rinsed with water and further treated as in example 2, however, the alkali silicate solution contained, instead of malonic acid, 2 percent sodium oxalate, based on the SiO, content.

EXAMPLE 4 Aluminum sheets were treated for 3 minutes at 85 C. with a solution containing 2.8 percent acid zinc phosphate, 2.5 percent phosphoric acid (75 percent solution) l percent chromic acid, and 0.5 percent nonionic wetting agent. Subsequently, the sheets covered with a phosphate coating were rinsed with water and heated for 2 minutes at 60 C. with a sodium silicate solution, as described in example 1 (A). The sodium silicate solution contained also an addition of 3 percent of potassium phthalate, based on the SiO, content. Afterwards, the so treated sheets were rinsed with water and dried. They showed good corrosion protection.

EXAMPLE 5 Deep-drawn sheets were coated with a phosphate layer by treating the sheets with a phosphatizing solution which contained 4 g. of calcium, l6.8 g. of sodium, 24 g. of 150., and 45.2 g. of NO, per liter. The treatment time was l0 minutes at a temperature of 98 C. The pH of the solution was 2.62. Subsequently, the sheets covered in this way with a calcium phosphate coating were rinsed with water and treated for 3 minutes at 55' C. with an alkali metal silicate solution as described in example 1 (C). The alkali metal silicate solution also contained an addition of 5 percent of maleic acid, based on the SiO, content, in the form of the ammonium salt. Afterwards, the sheets were rinsed with water and dried. Good corrosion protection was obtained.

EXAMPLE 6 Steel sheets were treated for 3 minutes at approximately 45 C. with a phosphatizing solution which contained 142 g. of Nil-5P0 42 g. of H,PO,, 1 18 g. of NaClO; and 15 g. of NaNO, per liter, and was diluted with water to a bath concentration of 4 volume percent. The pH of the l solution was adjusted with sodium carbonate to 4.5. Afterwards, the steel sheets, covered in this way with an iron phosphate coating, were rinsed with water and then treated for 90 seconds at 90 C. with an alkali metal silicate solution as described in example l (E). The sheets were then rinsed with water and dried. They had good corrosion protection.

The posttreating effect was still improved if the alkali metal silicate solution described above also contained an addition of approximately 5 percent by weight, based on the SiO, content, of a polybasic carboxylic acid in the form of its sodium salt of the compounds listed in table 2 of example l (G EXAMPLE 7 Several iron sheets were treated for 5 minutes in a phosphatizing solution at C. This phosphatizing solution contained 4.5 g. ZnO, 3.9 g. Na, 8.5 g. [50,, 14.9 g. "NO, and 0.l g. NO, per liter. Subsequently, the sheets, presently zinc phosphate coated, were rinsed with water and were treated I minute at 100 C. with a sodium silicate solution, whose SiO, concentration was 0.62 percent by weight and whose mol proportion SiO,:Na,O was 8.6:l. The sodium silicate contained 8 percent by weight, based on the SiO, content, of tetraethylammonium hydroxide. After drying, the sheets were exposed 24 hours to an atmosphere saturated with water vapor at a temperature of 50 C. After this operation, the phosphate layer was practically free of rust film and of rust stains.

Some of the phosphatized sheets were posttreated with a dilute 0.05 percent by weight chromic acid solution instead of with the sodium silicate solution, and likewise submitted to a 24-hour treatment at 50' C. in an atmosphere saturated with water vapor. After this treatment the phosphate layer showed rust film and rust stains.

EXAMPLE 8 Some iron sheets were coated with a zinc phosphate layer as described in example 7. After drying the sheets, a posttreatment with a sodium silicate solution in which the SiO, concentration was 0.04 percent by weight, the mol proportion of SiO,:Na, was 8.6:l, and which contained 8 percent by weight based on the SiO, content of tetraethylammonium hydroxide was carried out for seconds at 90 C. Subsequently, the sheets were dried without prior rinsing and lacquered with a commercial chlorinated rubber lacquer. In the same manner, similar phosphatized sheets, which had not been submitted to a treatment with a sodium silicate solution, were lacquered with the chlorinated rubber lacquer.

These sheets were exposed to an atmosphere saturated with water vapor, at a temperature of 50 C., and were examined after 7 days. The iron sheets posttreated with the sodium silicate solution, in the above-described manner, showed in comparison, to the untreated sheets, no wrinkle or bubble formation of the lacquer coating.

EXAMPLE 9 As described in example 7, several iron sheets were supplied with a zinc phosphate coating. Subsequently, part of the sheets were posttreated with a sodium silicate solution, whose SiO,

concentration amounted to 0.16 percent by weight and whose mol proportion of SiO,:Na,O amounted to 7:l. This solution contained 6.5 percent by weight, based on the SiO, content, of tetraethylammonium hydroxide. The remainder of the sheets were treated with a sodium silicate solution of the same SiO, concentration, however, with a mol proportion of SiO,:Na,O of 3.921. The sheets, dried without any interim rinsing, were subsequently exposed to an atmosphere saturated with water vapor at a temperature of 50 C. for 24 hours. No rust film or rust stains occurred with the sheets treated with a sodium silicate whose mol proportion of SiO,:Na O was 7:1. However, the other sheets showed considerable rust formation.

EXAMPLE Iron sheets coated with a zinc phosphate layer, as described in example 7, were posttreated with a potassium silicate solution whose mol proportion of SiO,:K,O was 6:1, and whose concentration was 0.02 percent SiO,. This solution contained 4.5 percent by weight, based on the SiO, content, of trimethylethanol ammonium hydroxide. Subsequently, the dried sheets were lacquered with a commercial chlorinated rubber lacquer and exposed 7 days to an atmosphere saturated with water vapor at a temperature of 50 C. No wrinkle or bubble formation occurred in the lacquer.

EXAMPLE 1 1 Nine percent by weight, based on the SiO, content, of sodium oxalate (finely pulverized) were added to a sodium silicate solution having an SiO, content of 22 percent, a mol proportion of SiO,:Na,O of 7:1, and 5 percent by weight, based on the SiO, content, of hexamethyl-hexamethylene diammonium hydroxide. The mixture was stirred under heating until total dissolution. The solution obtained was diluted to a SiO, content of 0.1 percent. Several iron sheets were treated for 5 minutes in a hot phosphatizing solution at 80 C. which contained, per liter, 4.5 g. of ZnO, 3.9 g. of Na, 8.5 g. of R0,, 14.9 g. of HNO,, and 0.1 g. of NO, Subsequently, the sheets now coated with a zinc phosphate layer were rinsed with water and treated for 1 minute at a temperature of 60 to 65 C. with the described activated sodium silicate solution. The sheets did not show any rust according to the Kesternich test, DIN 50017, after 48 hours. Sheets postpassivated with a nonactivated alkali silicate solution at a temperature of 60 to 65 C. were 50 percent rusted. An electrophoretic varnishing of the sheets postpassivated with the activated silicate solution yielded an unobjectionable crater-free surface and excellent varnish adhesion.

EXAMPLE l2 A sodium silicate solution having 22 percent of SiO,, a mol proportion of Si0,:Na,O of 9:1, and an addition of 7 percent by weight of hexamethyl-hexamethylene diammonium hydroxide, was activated by adding 2.5 percent by weight, based on the SiO, content of sodium malonate, in the fonn of a saturated solution. Furthermore, 0.5 percent by weight, based on the SiO, content of potassium antimony tartratc, were added to the solution. Subsequently, the solution was diluted to a content of 0.02 percent SiO,. Phosphatized iron sheets, as in example ll, were postpassivated with this solution for 2 minutes at a temperature of 60 C. After drying, the sheets were tested according to ASTM in the salt spray test. After 24 hours the sheets showed 20 percent rust. For comparison sake, sheets postpassivated with a chromic phosphoric acid solution showed 70 percent rust. The chromic phosphoric acid solution contained 33 percent of phosphoric acid and 33 percent chromic acid, and was diluted to a concentration of 0.04 percent by volume.

EXAMPLE l3 A potassium silicate solution having a concentration of 22 percent of SiO,, a mol roportion of SiO,:K O of 5:l and an addition of 0.5 percent y weight based on the SiO, content,

of hexamethyl-decamethylene diammonium hydroxide was activated by the addition of 5 percent sodium phthalate and 5 percent sodium citrate, both based on the SiO, content, in the tom of their aqueous solutions. Furthermore, 01 percent of sodium benzoate based on the SiO, content was added to the solution. The solution was diluted to a content of 0.05 percent Si0,. As in example 11, phosphatized iron sheets were posttreated with this solution at a temperature of 60 C. for 2 minutes, then rinsed with water, and examined in the Kesternich apparatus. After 24 hours, the sheets show no rust. The same results were achieved if instead of sodium benzoate, potassium antimony tartrate, paranitrophenol or hydrazine hydrate were used.

The alkali metal silicate solutions with a high SiO, content employed in the above-mentioned examples were prepared according to the described methods, as indicated. However, practically the same results are obtained if equimolar amounts of other quaternary ammonium compounds of the type indicated in the description are employed.

The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, however, that other expedients known to those skilled in the art may be employed without departing from the spirit of the invention.

We claim:

1. A method of posttreatment of phosphatized metal surfaces which comprises applying to phosphate layers applied to metal surfaces, an aqueous alkali metal silicate solution having a mol proportion of SiO, to alkali metal oxide selected from the group consisting of Na,O and KO of from 5 to l to 10 to l and a SiO, concentration of from 0.0l percent to 1 percent by weight, said alkali metal silicate solution containing a watersoluble quaternary nitrogen compound having at least one nonhydroxylated alkyl group on the quaternary nitrogen atom selected from the group consisting of mono and poly quaternary nitrogen compounds, said quaternary nitrogen compound being present in a ratio of SiO, to quaternary nitrogen compound calculated as quaternary ammonium oxide of from about 35 to l to about l,000 to l, and drying said posttreated phosphatized metal surfaces.

2. The method of claim 1 wherein said mol proportion of SiO, to alkali metal oxide is from 6 to l to 9 to l.

3. The method of claim I wherein said silicate solution has an SiO, concentration of from 0.02 percent to 0.2 percent by weight.

4. The method of claim 1 wherein said silicate solution contains from about 0.5 percent to about 25 percent by weight, based on the SiO, content of the alkali metal silicate solution, of a water-soluble salt of an organic polycarboxylic acid.

5. The method of claim 4 wherein said water-soluble salt of said organic polycarboxylic acid is present in an amount of from 2 percent to 10 percent by weight, based on the SiO, content of said alkali metal silicate solution.

6. The method of claim 1 wherein said silicate solution contains from about 0.05 percent to 2 percent by weight, based on the SiO, content of the alkali metal silicate solution, of a water-soluble corrosion inhibitor.

7. The method of claim 1 wherein said silicate solution contains from about 0.5 percent to about 25 percent by weight of a water-soluble salt of an organic polycarboxylic acid and from about 0.05 percent to 2 percent by weight of a watersoluble corrosion inhibitor, both based on the SiO, content of the alkali metal silicate solution.

8. The method of claim 1 wherein said aqueous alkali metal silicate solution is applied to said phosphate layers applied to metal surfaces at a temperature of between about 20 C. and C. for a period of from about 30 seconds to about 5 minutes.

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2. The method of claim 1 wherein said mol proportion of SiO2 to alkali metal oxide is from 6 to 1 to 9 to
 1. 3. The method of claim 1 wherein said silicate solution has an SiO2 concentration of from 0.02 percent to 0.2 percent by weight.
 4. The method of claim 1 wherein said silicate solution contains from about 0.5 percent to about 25 percent by weight, based on the SiO2 content of the alkali metal silicate solution, of a water-soluble salt of an organic polycarboxylic acid.
 5. The method of claim 4 wherein said water-soluble salt of said organic polycarboxylic acid is present in an amount of from 2 percent to 10 percent by weight, based on the SiO2 content of said alkali metal silicate solution.
 6. The method of claim 1 wherein said silicate solution contains from about 0.05 percent to 2 percent by weight, based on the SiO2 content of the alkali metal silicate solution, of a water-soluble corrosion inhibitor.
 7. The method of claim 1 wherein said silicate solution contains from about 0.5 percent to about 25 perceNt by weight of a water-soluble salt of an organic polycarboxylic acid and from about 0.05 percent to 2 percent by weight of a water-soluble corrosion inhibitor, both based on the SiO2 content of the alkali metal silicate solution.
 8. The method of claim 1 wherein said aqueous alkali metal silicate solution is applied to said phosphate layers applied to metal surfaces at a temperature of between about 20* C. and 100* C. for a period of from about 30 seconds to about 5 minutes. 